Relative path support in debug info

[devhawk]

• Support relative paths in debug info documents array
• Support optional document-root debug info property

Note, this change is entirely backwards compatible with the previous version of NEP-19. documents array elements may be absolute or relative and document-root property is optional

Spec update to match https://github.com/neo-project/neo-devpack-dotnet/pull/770

cc @erikzhang @shargon

[devhawk]

Note, the associated dotnet devpack PR associated with this NEP change has already been accepted.

[devhawk]

@shargon @erikzhang can one of you merge this please?